The present invention relates to a power supply circuit of the stabilized, or at least regulated, voltage type, for an electric motor, for the purpose of maximizing the efficiency of the motor or of regulating its speed, particularly for an asynchronous electric motor, and especiallyxe2x80x94but not exclusivelyxe2x80x94a single-phase asynchronous motor.
The present invention also relates to an electric motor comprising a power supply circuit of the stabilized voltage type, as well as a method for controlling the supply voltage of an electric motor.
In the design of electric motors, it is assumed that the power supply is at a constant voltage, and the motor is designed with the object of optimizing the performance of the motor at the design voltage, typically ranging from 200 to 230 V, or having other values according to the mains voltage used in the country in which the motor is to be used.
In fact, the supply voltage is not constant, as a result of possible voltage fluctuations in the mains, and also as a result of small differences in voltage in the various mains systems of the individual countries in which the motor may be used. Typically, the mains voltage may be as much as 255 V. In order to enable a motor designed for a specific voltage to operate correctly in conditions where the voltage may vary by as much as several tens of volts, control circuits based on the principle of slicing, particularly triac and chopper control circuits, are used at present. These circuits have some disadvantages, including a high harmonic content of the supply voltage to the motor (particularly for triac controllers) and a high cost, owing to the necessity of using high-voltage components and complicated control circuits. The production of chopper control circuits also has the disadvantage of a certain structural complexity due to the necessity of using controlled switches which operate at a high frequency with respect to the mains frequency.
The object of the present invention is to provide a new type of control circuit for the power supply of a motor, particularly an asynchronous motor, which can be used to set or stabilize the supply voltage while avoiding the disadvantages of traditional slicing circuits.
A further object of the present invention is to provide a new method of controlling the power supply to an electric motor, particularly an asynchronous motor, which enables a stabilized supply voltage to be obtained without the disadvantages of the conventional methods.
More particularly, the object of the present invention is to provide a circuit and a control method which do not require expensive components, which avoid the use of high-frequency switching elements, and which do not require mains filters for the elimination of electromagnetic noise.
These and further objects and advantages, which the following text will make clear to a person skilled in the art, are essentially obtained by means of a circuit characterized in that it comprises means of measuring a difference between a mains voltage and a reference voltage, and means for generating an alternating correction voltage whose frequency is equal to the frequency of the mains voltage, and which is phase-shifted with respect to said mains voltage, the phase shift between the mains voltage and the correction voltage being proportional to the difference between the mains voltage and the reference voltage, and said correction voltage being added to the mains voltage.
Essentially, therefore, the invention Is based on the idea of subtracting from, or adding to, the mains voltage an alternating wave at low voltage (correction voltage), whose phase with respect to the mains voltage is controlled in such a way that the sum of the two voltages supplies a stabilized voltage to the motor.
In one practical embodiment, it is possible to generate the correction voltage by using a full bridge consisting of four controlled switches, whose switching generates the correction voltage, which in this case is a square-wave voltage. A control logic switches the controlled switches according to the difference between the mains voltage and the value of the stabilized voltage which is required for the motor. The bridge of controlled switches is connected, through a direct-current branch of the bridge, to a virtually continuous voltage source.
Although this virtually continuous voltage source may be a source supplied from the same mains voltage which has been suitably processed, in a particularly advantageous embodiment the continuous voltage is obtained by using the inductive energy of the motor. For this purpose, a storage device in which the inductive energy is stored during certain phases of the operating cycle, and transferred to the motor in the remaining phases, can be located in the direct-current branch of the bridge of controlled switches. In one practical embodiment, the storage device consists of a capacitor or a group of capacitors.
Further advantageous embodiments of the circuit according to the invention and of the corresponding motor, as well as of the motor power supply method, are indicated in the attached claims and will be described in greater detail below with reference to one embodiment.